Patents are a legal monopoly. To say a company has a monopoly just means that it is extremely difficult to compete. In search, the market naturally created a monopoly.

Not only does google have twice the market share of anyone else, they likely have more than twice the number of advertisers as well. This means they are making more money on each visitor. That's why Yahoo and Ask both had to outsource their ads to Google, even though they had large market shares. Blekko has cited this as a huge reason they are having trouble competing.

Creating a real search engine (not a meta-search like Duck Duck Go) from scratch is very expensive. Blekko started out with 700 servers. Until you get to a very high scale, the server costs are prohibitive.

IIRC, Google tends to have more "real" searches. A lot of Bing and Yahoo searches probably have the query string "Facebook" because people have it as the default engine, or are already on a Yahoo / MS property (like their web mail). A lot of these people will then type "Google" into the Bing / Yahoo search box, when they need to find something.